Turning to FIG. 1, an example of a Sallen-Key LPF 100 can be seen. The LPF 100 generally comprises resistors R1 and R2, capacitors C1 and C2, and amplifier 102. As shown, this LPF 100 is a second order, non-inverting active LPF with the following, simplified (i.e., using an ideal amplifier) transfer function:
                              H          ⁡                      (            s            )                          =                              μ                                          R                1                            ⁢                              R                2                            ⁢                              C                1                            ⁢                              C                2                                                                        s              2                        +                          s              ⁡                              (                                                      1                                                                  R                        1                                            ⁢                                              C                        1                                                                              +                                      1                                                                  R                        2                                            ⁢                                              C                        1                                                                              +                                      1                                                                  R                        2                                            ⁢                                              C                        2                                                                                            )                                      +                          1                                                R                  1                                ⁢                                  R                  2                                ⁢                                  C                  1                                ⁢                                  C                  2                                                                                        (        1        )            For LPF 100, it can be seen that there are three main contributors to the output noise, namely, resistors R1 and R2 and amplifier 102. Because both resistors R1 and R2 contribute to the in-band noise at the output, the resistance values for resistor R1 and R2 are generally limited, which may require increased values of capacitors C2 and/or C1 for a given low pass cutoff frequency. This increase in the values of capacitors C2 and/or C1 can result in a significant area penalty. Therefore, there is a need for a filter having reduced in-band noise and having greater flexability than traditionally Sallen-Key LPFs.
Some other conventional circuits are: U.S. Pat. No. 7,471,140; and U.S. Patent pre-Grant Publ. No. 2008/0204129.